The present invention relates to an infrared detection element using the temperature dependence of spontaneous polarization of ferroelectrics. As an infrared detection element generally used for consumer appliances, there is a pyroelectric infrared detection element using the characteristics of changing of spontaneous polarization due to temperature changes.
In the pyroelectric infrared detection element, infrared light supplied to an infrared detection element periodically interrupted by a chopper. According to temperature dependence of the infrared detection element synchronized with the interruption frequency by the chopper, the infrared detection element is charged. The charge is amplified by an amplifier and read as a current or a voltage, thus infrared light is detected.
An example of a conventional pyroelectric infrared detection element already put into practical use is shown in FIG. 1. A pyroelectric element is made of a ceramics sintered substance 31 of barium strontium titanate (hereinafter, abbreviated to BST) having a heat-insulating structure, in which the element is stuck onto a Si IC substrate 32 for reading via metallic bumps 33 so as to improve the sensitivity and response speed. In the pyroelectric element of the kind using ceramics, there is a limit to decrease a size of each element, since it is extremely difficult to control the thickness and horizontal length of the element to several tens xcexcm or less. There is also a problem particularly in the response speed because a heat capacity of the element is large, thereby making it difficult to provide a small and light sensor.
Further, to solve such problems of the infrared detection element using a pyroelectric substance, production of an infrared detection element using a thin pyroelectric film has been tried. An example of a conventional infrared detection element using a thin pyroelectric film is shown in FIG. 2. As shown in the drawing, an infrared detection element 44 composed of a thin polycrystalline BST film held by upper and lower electrodes 43 is formed on the surface of an Si substrate 41 via a thin amorphous SiN (silicon nitride) support film 42. To realize the heat-insulating structure, an open space 45 by anisotropic etching is formed at the back of the Si substrate 41.
However, a pyroelectric property of such a thin polycrystalline BST film, particularly ferroelectricity that is a basis for development of the pyroelectricity, is far inferior to that of BST ceramics sintered at a high temperature. Moreover, the thinner the film is made, the more degraded are the ferroelectricity and the pyroelectric property in the prior art. Namely, when a thin polycrystalline pyroelectric film is used, the infrared detection element can be made thin and small. There is, however, a problem arises that the detection sensitivity is greatly reduced.
To make rapid improvement in the ferroelectric property and the pyroelectric property of such a thin pyroelectric film, a method for using a thin single-crystalline pyroelectric film by epitaxial growth has been proposed (see Japanese Patent No. 3094753).
A manufacturing process of a conventional infrared detection element using the thin single-crystalline pyroelectric film is exemplary shown in FIG. 3. As shown in the drawing, a thin single-crystalline film 52 of lantern-added lead titanate (hereinafter, abbreviated to PLT) by epitaxial growth is prepared and patterned on a single-crystalline MgO substrate 51(A). A first polyimide film 53 for supporting, a lower electrode 54, and a second polyimide film 55 are sequentially deposited and patterned (B). The single-crystalline MgO substrate 51 is inversely bonded on an alumina substrate 56 processed beforehand via a metallic bump 57. On the alumina substrate 56, a lead wire 58 is formed(C). The single-crystalline MgO substrate 51 is removed by etching (D). A photo detector electrode 59 is formed on the exposed surface of the thin PLT film 52. Here, the first polyimide film 53 is partially removed, and the lower electrode 54 is exposed. Conductive paste 60 and the lead wire 58 are, then, electrically connected on the surface thereof, and the thin PLT film 52 is bonded onto the alumina substrate 56(E).
The infrared detector composed of a thin single-crystalline PZT film with the heat resistant support, which is manufactured through such a complicated process as described, proved that the sensitivity is far higher than that of a detector composed of a thin polycrystalline film. However, there are problems in the infrared detector that the manufacturing process is complicated, that the yield rate is low, and that the cost is high.
Having made a number of experiments and theoretical analyses to improve the performance of an infrared detector using such a thin single-crystalline pyroelectric film, the inventors have found for the first time limits or problems on use of the thin single-crystalline pyroelectric film supported in the air. The discovered problems will be described hereunder in detail.
BaTiO3, one of the ferroelectrics for example, shows a primary phase transition in a following manner as temperature rises from a low temperature; rhombohedral crystal rhombic crystalxe2x86x92tetragonal crystalxe2x86x92cubic crystal. Further, when the temperature becomes lower from a high temperature, a primary phase transition, from cubic crystal to tetragonal crystal occurs inversely. In the course of the phase transition from the cubic crystal to the tetragonal crystal due to the temperature decrease, a and b axes shrink by about 0.005 xc3x85 and c axis extends by 0.01 xc3x85, from their original length of 4.01 xc3x85 at the curie temperature. With the phase transition, spontaneous polarization of 18 xcexcC/cm2 is generated in the crystal.
This discontinuous change of the lattice constant in the phase transition of the ferroelectrics brings about a discontinuous change in the temperature property of the spontaneous polarization. The discontinuous changes of the spontaneous polarization with the temperature in the neighborhood of the Curie temperature make accurate temperature measurement unable in the neighborhood of the Curie temperature.
The present invention provides an infrared detection element, which eliminates a discontinuous primary phase transition of ferroelectrics based on the fact described, and enables the accurate temperature measurement even in the neighborhood of the Curie temperature.
Further, the present invention is made to provide an infrared detection device incorporating the infrared detection element.
The infrared detection element according to the present invention has a single-crystalline base layer with a thickness of 50 nm to 10 xcexcm having a principal surface, a first electrode layer formed on the principal surface of the single-crystalline layer, a ferroelectric layer which is formed on the first electrode layer and composed of a single-crystalline layer or a unidirectionally oriented layer with which distortion in a direction within a surface parallel to the principal surface of the single-crystalline base layer is elastically constrained, and a second electrode layer formed on the ferroelectric layer, wherein an amount of charge of the ferroelectric layer of the infrared detection element is detected as an electric signal from the first and second electrode layer.
In the infrared detection element according to the present invention, the elastic restriction on the ferroelectric layer is not sufficient when the thickness of the single-crystalline base layer is less than 50 nm. However, when the thickness of the single-crystalline base layer is more than 10 xcexcm, heat capacity becomes large and the response to infrared light thus becomes slow.
Further, in the infrared detection element of according to the present invention, it is preferable that the ferroelectric layer has the perovskite structure so that the amount of charge in the ferroelectric layer may change depending on the temperature change caused by irradiation of infrared light.
Further, in the infrared detection element according to the present invention, it is preferable that the ferroelectric layer has a main component of Ba1-xSrxTiO3 (0xe2x89xa6xxe2x89xa61).
Further, in the infrared detection element according to the present invention, the first electrode layer is a noble metal layer such as Au, Pt or a conductive oxide layer of the perovskite structure and having a thickness of preferably 1 xcexcm or less, thereby effectively transferring the elastic restrictive force of the single-crystalline layer to the ferroelectric layer.
Further, in the infrared detection element according to the present invention, it is preferable that the single-crystalline layer is a single-crystalline silicon layer having a surface oriented to (001) direction and the ferroelectric layer is grown so as to have a surface oriented to (001) direction through an epitaxial growth or any other manufacturing method.
Further, in the infrared detection element according to the present invention, it is preferable that the a and b axes of the ferroelectric layer are formed within a surface parallel to the principal surface of the semiconductor layer and the c axis of the ferroelectric layer is formed in a direction perpendicular to the principal surface of the semiconductor layer.
Further, the infrared detection element according to the present invention has a single-crystalline base layer with a thickness of 50 nm to 10 xcexcm having a principal surface, a first electrode layer formed on the principal surface of the single-crystalline layer, a ferroelectric layer which is formed on the first electrode layer and composed of a first single-crystalline layer or a undirectionally oriented layer that the distortion in a surface parallel to the principal surface of the single-crystalline base layer is elastically constrained by the single-crystalline base layer, an infrared detection element having a second electrode layer formed on the ferroelectric layer, a third electrode layer formed on the principal surface of the single-crystalline layer, a second ferroelectric layer which is formed on the third electrode layer and composed of a single-crystalline layer or a unidirectionally oriented layer with which distortion in a direction within a surface parallel to the principal surface of the single-crystalline base layer is elastically constrained, a reference cell having a fourth electrode layer formed on the ferroelectric layer, and an infrared reflection film formed on the reference cell, wherein a charge of the ferroelectric layer of the infrared detection element is detected as an electric signal from the first and second electrode layer, a charge of the ferroelectric layer of the reference cell is detected as an electric signal from the third and fourth electrode layer, and the difference between the electric signals is detected.
Further, the infrared detection element according to the present invention has a single-crystalline base layer with a thickness of 50 nm to 10 xcexcm having a principal surface, a first electrode layer formed on the principal surface of the single-crystalline layer, a first ferroelectric layer composed of a single-crystalline layer which is fixed to the first electrode layer, an infrared detection element having a second electrode layer formed on the ferroelectric layer, a third electrode layer formed on the principal surface of the single-crystalline base layer, a second ferroelectric layer composed of a single-crystalline layer which is fixed to the third electrode layer, a reference cell having a fourth electrode layer formed on the ferroelectric layer, and an infrared reflection film formed on the reference cell, wherein a charge of the ferroelectric layer of the infrared detection element is detected as an electric signal from the first and second electrode layer, a charge of the ferroelectric layer of the reference cell is detected as an electric signal from the third and fourth electrode layer, and the difference between the electric signals is detected.